Storage devices such as FC (Fibre) disks, SATA (Serial Advanced Technology Attachment) disks, SAS (Serial Attached SCSI) disks, SSDs (Solid State Drives), and optical disks are used in a storage system. When these storage devices are used, the resulting performance of volumes constituting the storage system will be different depending on which type of storage devices is used.
For example, if SSDs are used as storage devices, they can constitute volumes that are expensive and exhibit high response performance. On the other hand, if SATA disks are used as storage devices, they can constitute inexpensive volumes although they exhibit lower response performance than the volumes constituted from SSDs; and it is possible to secure a larger capacity at the same cost as that of the SSDs.
Also, data stored in the volumes have a tendency to be used frequently by users immediately after they are stored in the volumes; but they will be used less frequently as time passes. If data not used remain to be stored in high-performance volumes, this will result in wasteful consumption of the high-performance volumes.
Therefore, a data management method called a Hierarchical Storage Management (HSM) function as described in, for example, Japanese Patent Application Laid-Open (Kokai) Publication No. 2010-97359 is suggested. This HSM function is implemented by, for example, a storage system having a plurality of volumes with different performance, where a file shared server creates a file system in each volume. In this circumstance, the file shared server migrates a file from a high-performance volume to a low-performance volume according to file usage frequency.
Specifically speaking, a plurality of storage tiers (Tiers) are configured according to the performance of storage devices; and a frequently used file(s) is retained in Tier1 constituted from high-speed, expensive volumes, and a file(s) which is not used so often is retained in Tier2 constituted from low-speed, inexpensive volumes. If a policy is set under the circumstances described above to migrate a file belonging to Tier1 to Tier2 after the elapse of a certain period of time since last access, a file belonging to Tier1 is migrated to Tier2 according to the description of this policy. In this case, data can be migrated in files, which are small units. Therefore, it is easy to migrate a file between the storage tiers and it is possible to change the storage location of a file depending on the performance, cost, and characteristics of the storage devices. Furthermore, if a lower-level storage tier (Tier3) having non-rewritable media called WORM (Write Once Read Many) is configured, it is possible to achieve a balance between the performance and cost and guarantee no falsification of data.